1. Field of the Invention
The present invention relates to a projection exposure apparatus used for forming fine patterns in, for example, semiconductor integrated circuits, liquid crystal displays, etc. More particularly, the present invention relates to a projection exposure apparatus having a mechanism for maintaining the image-forming performance of its projection optical system in a favorable condition.
2. Description of the Related Art
A photolithography process for forming a circuit pattern in a semiconductor device or the like uses a projection exposure apparatus (e.g., stepper) in which a pattern formed on a mask (a reticle) is transferred to a photosensitive substrate (e.g., a semiconductor wafer, glass plate, etc.), which has been coated with a photoresist, through a projection optical system. The projection optical system of such a projection exposure apparatus is incorporated in the apparatus after high-level optical designing, careful selection of a vitreous material, superfine processing of the vitreous material, and precise assembly adjustment. The present semiconductor manufacturing process mainly uses a stepper in which a reticle (or a photomask, etc.) is irradiated with the i-line (wave-length: 365 nm) of a mercury-vapor lamp as illuminating light, and light passing through a circuit pattern on the reticle is passed through a projection optical system to form an image of the circuit pattern on a wafer (or a glass plate, etc.), which has been coated with a photoresist. An excimer stepper that employs an excimer laser (KrF laser of wavelength 248 nm) as an illuminating light source has also been used for evaluation or research purposes.
With the steady increase of the degree of integration of VLSI and other similar devices, various methods have been developed for projection exposure apparatuses in order to perform transfer of finer patterns, such as optimization of illuminating conditions, new schemes of exposure method, etc. For example, there has been proposed a method of improving the resolution and DOF (Depth of Focus) by previously obtaining the most suitable combination of a coherence factor of the illuminating optical system (i.e., value: the ratio of the numerical aperture (N.A.) of the illuminating optical system to the numerical aperture of the projection optical system) and the numerical aperture of the projection optical system for each specific pattern line width, and selecting the most suitable combination for each pattern line width.
Among projection exposure apparatuses which are presently put to practical use, those which are designed for the i-line include a projection optical system having a numerical aperture (NA) of about 0.6. In general, for the same wavelength of illuminating light used, as the numerical aperture of the projection optical system is increased, the resolution improves correspondingly. However, as the numerical aperture NA increases, the focal depth DOF becomes shallower in proportion to .lambda./NA.sup.2, where .lambda. is the wavelength of illuminating light.
Incidentally, the resolution can be improved by increasing the image-side numerical aperture NAw (cf. the object-side numerical aperture NAr) of the projection optical system. That is, the resolution can be improved by increasing the pupil diameter of the projection optical system and also increasing the effective aperture of an optical element, e.g., lens, which constitutes the projection optical system. However, the focal depth DOF decreases in inverse proportion to the square of the numeral aperature NAw. Accordingly, even if a projection optical system of high numerical aperature can be produced, the required focal depth cannot be obtained; this is a considerable problem practical use.
Assuming that the wavelength of illuminating light is 365 nm of the i-line and the numerical aperature NAw is 0.6, the focal depth DOF decreases to about 1 .mu.m (.+-.0.5 .mu.m) in total range. Accordingly, a resolution failure occurs in a portion whenever the surface unevenness or the curvature is greater than DOF within one shot area (which is about 20 by 20 mm to 30 by 30 mm square) on the wafer.
In order to cope with these problems, the following various methods have been devised:
First, super-high resolution techniques, e.g., an annular zone illuminating method, modified light source method, phase shift method, etc., have been proposed. Among them, the annular zone illuminating method is a technique whereby the light intensity distribution of an illuminating light beam in a pupil plane of an illuminating optical system or a plane neighboring it is regulated to an annular zone shape, and a reticle pattern is illuminated with such an illuminating light beam, as disclosed in Japanese Patent Application Public Disclosure (KOKAI) No. Sho 61-91662. The modified light source method (also known as SHRINC method or inclined illuminating method) is a technique whereby the light intensity distribution of an illuminating light beam in a pupil plane of an luminating optical system or a plane neighboring it is made maximum at least at one position that is a predetermined amount off from the optical axis of the illuminating optical system, and thus the illuminating light beam is applied to a reticle pattern at a predetermined angle of inclination, as disclosed in Japanese Patent Application Public Disclosure (KOKAI) Nos. Hel 04-101148, Hel 04-180612, Hel 04-225358, Hel 04-180613 and Hei 04-225514.
In regard to these problems, super-high resolution techniques have been proposed, for example, a phase shift method such as that disclosed in Japanese Patent Application Post-Exam Publication No. Sho 62-50811, and a SHRINC (Super High Resolution by Illumination Control) method disclosed, for example, in W092/03842, Japanese Patent Application Disclosure (KOKAI) No. Hei 04-180612 and Japanese Patent Application Disclosure (KOKAI) No. Hei 04-1801613 (corresponding to U.S. Ser. No. 791,138 filed on Nov. 13, 1991).
The phase shift method is carried out by using a phase shift reticle having a phase shifter (e.g., a dielectric thin film) whereby the phase of light passing through a specific one of light-transmitting portions of a circuit pattern formed on the reticle is shifted by .pi.[grad] with respect to the phase of light passing through another light-transmitting portion, as disclosed. The use of such a phase shift reticle enables the resolution to be improved in comparison to the use of an ordinary reticle (i.e., a conventional reticle composed only of a light-transmitting portion and a light-blocking portion) for a predetermined pattern. It should be noted that typical phase shift reticles include a spatial frequency modification type (Japanese Patent Application Publication No. Sho 62-50811), a half-tone type (Japanese Patent Application Public Disclosure (KOKAI) No. Hei 04-162039), a shifter shielding type, and an edge enhancement type.
However, none of the above-described methods are effective for all reticle patterns, that is, all line widths and configurations. Therefore, it is necessary to select an illuminating method and conditions which are most suitable for each reticle or reticle pattern. Accordingly, the projection exposure apparatus needs to have a structure which enables illuminating conditions (v value and other conditions) in the illuminating optical system to be varied. For example, when the phase shift method is used, it is necessary to optimize the a value of the illuminating optical system.
Further, with the above-described methods, advantages such as an improvement in the resolution and an increase in the focal depth can be effectively obtained when a circuit pattern to be transferred is a periodic pattern having a relatively high density. However, substantially no effect can be obtained for discrete patterns (isolated patterns) such as those called "contact hole patterns".
To enlarge the apparent focal depth for isolated patterns, e.g., contact hole patterns, an exposure method has been proposed, for example, in U.S. Pat. No. 4,869,999, in which exposure for one snow area on a wafer is carried out in a plurality of successive exposure operations, and the wafer is moved along the optical axis of the projection optical system by a predetermined amount during the interval between each pair of successive exposure operations. This exposure method is called FLEX (Focus Latitude enhancement Exposure) method and provides satisfactory factory focal depth enlarging effect for isolated patterns, e.g., contact hole patterns. However, since the FLEX method indispensably requires multiple exposure of contact hole pattern images which are slightly defocused, a resist image obtained after development inevitably lowers in sharpness (the rise of the edge of the resist layer).
There has also been proposed a technique whereby the focal depth is increased during projection of contact hole patterns by a method different from the FLEX method wherein the wafer is moved along the optical axis during the exposure operation. In the Super-FLEX method published in Extended Abstracts (Spring Meeting, 1991) 29a-ZC-8, 9, Japan Society of Applied Physics, a phase filter having a concentric amplitude transmittance distribution centered at the optical axis is provided on the pupil plane (i.e., a Fourier transform plane with respect to the reticle) of the projection optical system so as to increase the effective resolution and focal depth of the projection optical system by the action of the filter.
The method wherein the transmittance distribution or phase difference is changed by filtering at the pupil plane of the projection optical system to thereby improve the focal depth as in the case of the Super FLEX method is generally known as "multifocus filter method". The multifocus filter method is detailed in the paper entitled "Study of Imaging Performance of Optical System and Method of Improving the Same", pp.41-55, in Machine Testing Institute Report No. 40, issued on Jan. 23, 1961. The method of improving the image quality by spatial filtering at the pupil plane is generally called a "pupil filter method". The assignee has proposed, as a new type of filter usable for such pupil filter method, a filter of the type that blocks light only in a circular area in the vicinity of the optical axis (this filter will hereinafter be referred to as "light-blocking pupil filter") in Japanese Patent Application Public Disclosure (KOKAI) No. Hei 04-179958. The assignee has further proposed a pupil filter method named "SFINCS method" that uses a pupil filter designed to reduce the spatial coherence of a bundle of image-forming rays from a contact hole pattern which passes through the pupil plane in U.S. Pat. application Ser. No. 128,685 (Sep. 30, 1993).
Separately from the above-described pupil filters for contact hole patterns, pupil filters which are effective for relatively dense periodic patterns, e.g., line and space (L&S) patterns, have also been reported, for example, in "Projection Exposure Method Using Oblique Incidence Illumination: Principle" (Matsuo et al.: 12a-ZF-7) in Extended Abstracts (Autumn Meeting, 1991), Japan Society of Applied Physics, and in "Optimization of Annular Zone Illumination and Pupil Filter" (Yamanaka et al.: 30p-NA-5) in Extended Abstracts (Spring Meeting, 1992), Japan Society of Applied Physics. These filters are adapted to reduce the transmittance (i.e., the transmitted light intensity) of a circular or annular area centered at the optical axis (this type of filter will hereinafter be referred to as "filter for L&S patterns"). In the L&S pattern filter method, the phase of light passing through the filter is not changed, unlike in the Super FLEX method.
Incidentally, the exposure apparatus is required to provide not only high resolution but also high alignment accuracy in formation of fine patterns of semiconductor integrated circuits, etc. That is, patterns of successive layers must be transferred such that the pattern of the subsequent layer is accurately superimposed on the pattern of the preceding layer. Accordingly, the exposure apparatus is required not only to perform accurate detection of alignment marks on the wafer and accurate alignment between the reticle and the wafer but also to use a projection optical system having minimal distortion. It is assumed that the distortion includes not only ordinary barrel form distortion and pincushion distortion but also random variation of the image position caused mainly by possible manufacturing errors of lens elements.
Among various exposure methods using pupil filters, the Super FLEX method, the light-blocking pupil filter exposure method and the SFINCS method enable the resolution and focal depth to be effectively increased with respect to isolated contact hole patterns among fine patterns which are to be transferred by exposure, as described above. However, for relatively dense (periodic) patterns, e.g., L&S patterns, these methods cause the resolution to lower undesirably. Therefore, when L&S patterns or other relatively dense patterns are to be exposed, it is necessary to unload the pupil filter from the projection optical system or to exchange it for a filter for L&S patterns.
As has been described above, the projection optical system is completed through a combination of high-level designing and production, together with strict adjustment, to obtain a favorable projected image. Accordingly, if the pupil filter, which changes the optical characteristics of the bundle of image-forming rays, is merely loaded, unloaded or exchanged, the image-forming characteristics of the projection optical system are undesirably changed and cannot accurately be maintained at the desired level.
In the case of an exposure apparatus designed on the premise that it will be used only for specific patterns, e.g., contact hole patterns, the projection optical system may be adjusted with a specific pupil filter incorporated thereinto when the system is set up, as a matter of course. However, the above-described problems inevitably arise in such a case where a single exposure apparatus is used for pattern transfer by exposure at various steps in order to increase the production efficiency as in the case of the present production lines for semiconductor devices or the like.
Further, there may be cases where exposure is carried out by combining together information as to whether or not a pupil filter is present and about the type of pupil filter used, and the change of illuminating conditions (i.e., change of the .sigma. value or use of annular zone illumination, etc.). In such cases, the condition of variation of the image-forming characteristics changes under each set of conditions. The condition of variation of the image-forming characteristics also changes when the pupil filter method and a conventional high-resolution technique are employed in combination. When such a change of the image-forming characteristics is corrected through a correcting mechanism using parameters corrected as described above, no problem will arise from the long-term standpoint. However, there is a problem that the image-forming characteristics have past hysteresis on account of the phenomenon of heat accumulation in the projection optical system. Accordingly, when the illuminating conditions or the pupil filters are changed from one to another according to the type of reticle or reticle pattern, even if the amount of change of the image-forming characteristics is calculated and the characteristic change is corrected immediately on the basis of the parameters corrected under the new conditions, the image-forming characteristics cannot accurately be corrected as long as the Hysteresis according to the previous conditions remains in the projection optical system. This problem may occur in the following two forms:
Firstly, owing to the distribution of heat generated under the illuminating conditions before the change of the operating conditions and by the pupil filter used under these conditions, image-forming characteristics obtained under the new illuminating conditions and pupil filter (used after the condition change) do not coincide with the actual image-forming characteristics even if they are obtained by taking into consideration an offset component attendant on the change of the conditions. That is, since the offset component is determined under conditions where the projection optical system is not affected by the absorption of illuminating light, if the influence of the absorption of illuminating light before the change of the conditions remains, it is necessary to additionally give an offset corresponding to the influence of the absorption of illuminating light. In other words, since the amount of change of the image-forming characteristics becomes discontinuous before and after the change of illuminating conditions and pupil filters, the image-forming characteristics cannot be accurately corrected continuously when the illuminating conditions, together with the pupil filters, are changed from one to another.
Secondly, even if the first problem is solved by some method, a second problem arises from the exposure carried out under the new illuminating conditions and pupil filter. That is, immediately after the change of illuminating conditions and pupil filters, the heat distribution condition under the previous conditions and that under the new conditions overlap each other, forming a state of being neither of the two heat distribution conditions, at a lens element in the vicinity of a pupil plane of the projection optical system. Accordingly, even if an amount of change of the image-forming characteristics is calculated on the basis of the parameters under either of the illuminating conditions, the result of the calculation is not coincident with the actual amount of image-forming characteristic change. The image-forming characteristics (i.e., the heat distribution condition in the projection optical system) in such a transient state cannot be expressed simply by a sum of the characteristics before and after the change of illuminating conditions and pupil filters, and it is extremely difficult to calculate and correct a change of the image-forming characteristics in the transient state.